Food-grade delivery system for controlled gene expression in Lactococcus lactis.

A food-grade system for the delivery of desired genes to Lactococcus lactis, their inducible expression, and their transfer to related strains was established. Based on the thermosensitive pG(+)host replicon, two types of plasmid vectors were constructed which contained sections of either the chromosomal leu operon of L. lactis or the tel operon from the lactococcal sex factor. Genes cloned into the leu or tel sequences of these vectors were delivered to the homologous regions of the chromosome or the sex factor through two single crossovers, leading to integration of the recombinant plasmids and subsequent excision of the vector portions. Inducible transcription of integrated genes was achieved by using the nisin-controlled expression (NICE) system. To establish the signal transduction genes nisRK in L. lactis, the vectors pLNG1363 (targeted to the chromosome) and pUK500 (targeted to the sex factor) were constructed. Fusions of six different peptidase genes (pep) from Lactobacillus delbrueckii with the nisin-inducible promoter P(nisA) were delivered to the sex factor with derivatives of the vector pUK300. Food-grade recombinants of L. lactis were constructed which had the nisRK genes and individual P(nisA)::pep fusions integrated either separately into the chromosome and the sex factor or simultaneously into the sex factor. With both types of recombinants, expression of P(nisA)::pep fusions after induction with nisin was demonstrated. Depending on the loci used for integration of nisRK, variable induction rates were observed. Furthermore, an engineered sex factor carrying a P(nisA)::pepI fusion was transfered by conjugation between two strains of L. lactis at a frequency of 4 x 10(-4).